1. Field of the Invention
The present invention relates to a phase shifting mask (PSM) and in particular to a simulation based repair technique for clear defects on an attenuated PSM.
2. Discussion of the Related Art
Photolithography is a well-known process used in the semiconductor industry to form lines, contacts, and other known structures in integrated circuits (ICs). In conventional photolithography, a mask (or a reticle) having a pattern of transparent and opaque regions representing such structures in one IC layer is illuminated. For example, FIG. 1 illustrates a cross section of a binary mask 100 having a transparent substrate 101 and a plurality of opaque (e.g. chrome) regions 102 formed on transparent substrate 101. During an exposure operation, a radiation source 103 is directed at mask 100. The emanating light from mask 100 is then focused on a photoresist layer provided on a wafer. During a subsequent development process, portions of the photoresist layer are removed, wherein the portions are defined by the pattern. In this manner, the pattern of mask 100 can be transferred to or printed on the photoresist layer.
FIG. 2 illustrates an attenuated phase shifting mask (PSM) 200 that uses phase destructive interference of the waves of incident light to transfer its pattern. Generally, phase-shifting shifts the phase of a first region of incident light waves approximately 180 degrees relative to a second, adjacent region of incident light waves to create a sharply defined interface between the first and second regions. For example, PSM 200 includes clear regions 202 and attenuated phase shifting regions 201. The phase shift of light passing through an attenuated phase shifting region 201 relative to light passing through a clear region 202 is approximately 180 degrees. Clear regions 202 are transparent, i.e. regions having an optical intensity transmission coefficient T>0.9. In contrast, attenuated phase-shifting regions 201 are partially transparent, i.e. regions having a low optical intensity transmission coefficient 0.03<T<0.2.
Unfortunately, because of the difficulty in providing a perfect mask fabrication process, at least some defects can be found on most masks. In general, a defect on a mask is anything that is different from the design database and is deemed out of tolerance by an inspection tool or an inspection engineer. Common mask defects include, for example, an isolated opaque pinhole defect in a clear region, an isolated clear spot defect in an opaque/attenuated region, an edge intrusion defect in an opaque/attenuated region, an edge protrusion defect in a clear region, a geometry break defect in an opaque/attenuated region, and a geometry bridge defect in a clear region.
Repairing defects in the clear regions, i.e. removal of material from regions in which the material does not belong, can be easily and accurately performed on both the binary mask as well as the attenuated PSM with a focused laser beam.
FIG. 4A illustrates a conventional process 400 for repairing clear defects on a generic (e.g. a binary or an attenuated PSM) mask. In process 400, after writing the mask with the desired pattern, the mask can be inspected in step 401. The inspection can include scanning the surface of the mask with a high-resolution microscope and capturing images of the mask. Step 402 determines whether a clear defect is found. If so, then step 403 determines whether the printing of this clear defect is not out of tolerance. Note that the goal of inspection is to correctly identify a defect to avoid a failed wafer processing. However, not all mask defects are important with respect to the desired result, i.e. an accurate representation of the original design layout on the photoresist material or etched into silicon. Specifically, not all mask defects will “print.” The printability of a defect is how a defect would impact the outcome of a given photolithography and/or etching process. If the printing is not out of tolerance, then the mask can be approved (called CD registration) in step 405.
However, if the printing of the clear defect is out of tolerance, then step 404 determines whether that clear defect can be repaired. Note that such a designation might be contingent on the amount that the defect is out of tolerance or number of clear defects found in close proximity to each other. For example, the more defects found adjacent the clear defect, the greater the probability that the mask cannot be repaired in a cost efficient manner. Specifically, if clear defects are close to each other, then the aerial images of the patterns are correlated when they are repaired. This correlation makes the repair process control more complex, thereby increasing the difficulty of repair. Moreover, if the total number of defects on the mask is more than a predetermined amount, then repair of the mask is also not cost effective. If the clear defect is not repairable, then the mask is disqualified in step 406.
If the clear defect is repairable, then material can be deposited in the area identified as the clear defect in step 407. For example, FIG. 4B illustrates one repair solution to attenuated PSM 300. Specifically, in repaired mask 410, clear defect 302 has been filled in using a “plug” 411. Note that the exterior edges of plug 411 are substantially collinear with the edges of line 301. After cleaning the mask in step 408, process 400 can return to step 402 to identify other clear defects, if present. Otherwise, process 400 can end.
Of importance, sputtering tools, which are used to repair clear defects on masks (i.e. adding of material to regions in which the material does belong), can easily and accurately deposit carbon, but not other materials. Thus, neither chrome, which is typically used to form opaque regions 102 in binary mask 100 (FIG. 1), nor molybdenum silicide, which is typically used to form attenuated phase shifting regions 201 in attenuated PSM 200, can be used to repair clear defects.
The use of carbon to repair a clear defect on a binary mask is satisfactory because the intensity transmission coefficient of the opaque (e.g. chrome) regions and of the deposited carbon is identical, i.e. T<0.01. Thus, the repair solution in the binary mask accurately emulates a defect-free clear region. However, the use of carbon to repair a clear defect on an attenuated PSM is sub-optimal because the intensity transmission coefficient of the attenuated regions is significantly higher than for the deposited carbon. For example, the intensity transmission coefficient of molybdenum silicide is approximately T=0.03-0.2. Thus, carbon would fail to provide the desired phase shift on the attenuated PSM.
Without the appropriate phase shift, the printing of a feature on a wafer using attenuated PSM 410 may be out of tolerance. For example, FIG. 5 illustrates an aerial image 500 of a wafer printed using attenuated PSM 410. Note that printed line 501, which corresponds to attenuated line 301 and carbon plug 411 (see FIG. 4B), has not accurately printed. Specifically, printed line 501 is thinner at location 502, which is the area associated with carbon plug 411.
To address this problem, mask shops can collect experimental data on various plug shapes and sizes by exposing a test mask and then analyzing the resulting printed features on the developed wafer. This experimental data can be expanded to include multiple test masks exposed under various stepper scanner conditions, which can significantly impact the printability of such features on the wafer. These conditions can include: wavelength, numerical aperture, coherence factor, illumination mode, exposure time, and exposure focus/defocus among others. Unfortunately, the collection of this experimental data requires the use of expensive mask writing and exposure tools, valuable blank wafers (which could otherwise be used for integrated circuits), as well as the time-consuming analysis of a skilled engineer, thereby rendering an empirical approach impractical for commercial mask production.
Therefore, when asked to propose a repair solution for a clear defect on an attenuating PSM, an engineer may attempt to compensate for the lack of phase effect by arbitrarily increasing the plug size a predetermined amount for all clear defects on the mask. For example, FIG. 6 illustrates another repair solution to attenuated PSM 300. Specifically, repaired mask 600 includes a carbon plug 601 that is significantly larger than plug 411 (FIG. 4B). However, without the benefit of empirical data, the engineer cannot know whether the size and/or shape of plug 601 will result in a printed feature within tolerance. This uncertainty has contributed to the reluctance of many mask shops to use attenuated PSM technology, despite the fact that an attenuated PSM can provide more smaller, more sharply defined features than a binary mask.
Therefore, a need arises for an automated technique to provide accurate repairs to clear defects on an attenuated PSM.